@ Emmanuel: You cannot tell DEVONthink to find a document only when both terms appear on the same line. You can, however, use Boolean operators such as NEAR to tell DEVONthink only to report a document when both words appear not more than ten words apart from each other. Use NEAR/n for even more granular control (find words that are n words apart).

Tags are not indexed so they aren’t searchable in this way.
You’d need to use a Smart Group or add Tags criteria in the Advanced section of Tools > Search.
Select ALL (which functions as AND) instead of ANY (which functions as OR).